This invention relates to the manufacture of glass optical fibres provided with coatings of relatively low water permeability.
One of the major problems in the long term mechanical durability of the glass optical fibres used in communication systems is the degradation with time of the high strength of the freshly drawn fibre. One of the principal causes of this degradation in strength is the result of stress corrosion, otherwise known as static fatigue, of the fibre. Minute cracks in the glass surface, which is typically of silica, are found to grow when the fibre is stressed in the presence of moisture. Water is found to play a part in the way in which such cracks progagate, and hence a reduction in the moisture level at the silica surface would prolong the life of the fibre.
Conventionally the pristine surface of freshly drawn optical fibre is protected by an on-line coating of plastics material. Typically this is a thin coating provided by solution coating, and is followed up by a secondary coating provided by extrusion. These coatings significantly reduce the rate at which the fibre strength is degraded by atmospheric attack, but there is considerable room for further improvement in view of the permeability to water of plastic layers deposited in this way.
Approaches have been made to provide improved resistance to water penetration by applying a metal coating to the glass prior to the application of any plastics coating. Such an approach has been reported for instance by D. A. Pinow et al in Applies Physics Letters, Volume 34(1) January 1979, pages 17 to 19, and by J. A. Wysocki et al in Advances in Ceramics, Volume 2 (Physics of Fibre Optics) B. Bendow, S. Mitron (Editors) ACS Inc. 1981 page 134. However, a drawback of this approach has been that in general any reduction in the rate of stress corrosion arising from improved hermeticity is accompanied by a reduction in the initial strength believed to be occasioned by damage done to the glass surface during the metal deposition.
An alternative approach is described in European Patent Application Specification No. 0034670 in which the glass optical fibre is provided with a primary plastics coating before the application of a metal coating on the plastics coating. This metal coating is applied from the melt, and hence considerable constraints are imposed upon the choice of compatible metals and plastics for this application.